In IEEE 802.11 wireless systems, channel sounding and feedback process is commonly used for channel estimation. In MIMO systems, each channel sounding and feedback process is followed by a series of MIMO frame exchange. During channel sounding and feedback, a transmitting device (initiator) sends a sounding announcement (e.g., null data packet announcement (NDPA)) followed by a sounding packet (e.g., null data packet (NPD)) to a receiving device (responder) participating in the process. The responder estimates the channel during the preamble portion of the sounding packet. The responder then feedbacks the average SNR (signal-to-noise ratio) and CSI (channel state information) to allow the initiator to compute the transmit antenna (precoding) weights for MIMO transmission. Feedback packet may also include other channel quality metrics such as MCS, BER, SNR/SINR, and mutual information.
Feedback of accurate channel quality information such as SNR and MCS allows the transmitter to make correct decision regarding transmission bandwidth adjustment as well as MCS adaptation to improve system performance. In current implementation, channel quality information is provided based on a fixed sub-channel (e.g., the sounding bandwidth) and obtained through a sounding and feedback protocol. The channel conditions, however, could be significantly different in different sub-channels due to frequency selective fading. To have channel quality information for all sub-channels, multiple requests and feedbacks are required. This leads to increased system overhead and channel contention.
In orthogonal frequency division multiple access (OFDMA) systems, frequency division multiple access is achieved by assigning different OFDM sub-channels to different users. OFDMA design can benefit from multiuser diversity gain. CSI feedback for multiple users are required to achieve multiuser diversity gain. However, the existing CSI/SNR feedback scheme does not work due to large feedback overhead. For example, the compressed CSI feedback has very large overhead. On the other hand, SNR feedback only provides feedback for average SNR over data subcarriers and space-time streams. It cannot provide SNR on sub-channels.
A solution is sought to provide sub-channel SNR feedback and CSI reporting with reduced overhead and channel contention.